Possible Observation of Quadrupole Waves in Spin Nematics
Phys. Rev. Lett. 135, 156704 – Published 9 October, 2025Erratum Phys. Rev. Lett. 136, 209901 (2026)
DOI: https://doi.org/10.1103/pywx-vxfh
Abstract
Discovery of new states of matter is a key objective in modern condensed matter physics, which often leads to revolutionary technological advancements such as superconductivity. Quantum spin nematic, a “hidden order” that evades conventional magnetic probes, is one such state. is a potential spin nematic material, suggested by the observation of a two-magnon Bose-Einstein condensation from above the saturation field. However, direct confirmation of the spin nematicity remains elusive. This Letter presents inelastic neutron scattering spectra from the putative spin nematic phases of , revealing low-energy quadrupole waves that are absent in the neighboring conventional magnetic phases. A spin-one model quantitatively captures the full details of the spin excitation spectra across all low-temperature phases, providing direct evidence of the spin nematic orders. Additionally, we show evidence of the three-magnon continuum and two-magnon bound states in the -magnetization plateau, revealing condensation of the two-magnon bound state as the origin of the low-field spin nematic supersolid phase.